1. health and fitness
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554802
research-article2014
JLAXXX10.1177/2211068214554802Journal of Laboratory AutomationSun et al.
Review Article
Advances in Techniques for Probing
Mechanoregulation of Tissue Morphogenesis
Journal of Laboratory Automation
2015, Vol. 20(2) 127­–137
© 2014 Society for Laboratory
Automation and Screening
DOI: 10.1177/2211068214554802
jala.sagepub.com
Jian Sun1*, Yuan Xiao1*, Shue Wang1*, Marvin J. Slepian2,3, and
Pak Kin Wong1,2
Abstract
Cells process various mechanical cues in the microenvironment to self-organize into high-order architectures during tissue
morphogenesis. Impairment of morphogenic processes is the underlying cause of many diseases; as such, understanding
the regulatory mechanisms associated with these processes will form the foundation for the development of innovative
approaches in cell therapy and tissue engineering. Nevertheless, little is known about how cells collectively respond
to mechanical cues in the microenvironment, such as global geometric guidance, local cell-cell interactions, and other
physicochemical factors, for the emergence of the structural hierarchy across multiple length scales. To elucidate the
mechanoregulation of tissue morphogenesis, numerous approaches based on biochemical, biomaterial, and biophysical
techniques have been developed in the past decades. In this review, we summarize techniques and approaches for probing
the mechanoregulation of tissue morphogenesis and illustrate their applications in vasculature development. The potential
and limitations of these methods are also discussed with a view toward the investigation of a wide spectrum of tissue
morphogenic processes.
Keywords
automated biology, microtechnology, nanotechnology
Introduction
Tissue morphogenesis is a fundamental multicellular activity that is essential for various developmental and regenerative processes. Vasculogenesis, for instance, leads to de
novo blood vessel formation from mesodermal precursors
overlying the endoderm.1 Organs of ectodermal origin, on
the other hand, are vascularized by angiogenesis—the outgrowth of new capillaries from the existing vasculature.2
The formation of new blood vessels also contributes to
numerous malignant, ischemic, inflammatory, infectious,
and immune disorders.3,4 Particularly, tissue engineering, a
rapidly emerging field in regenerative medicine, has great
potential to restore, maintain, or improve tissue functions.
Skin and cartilage tissue engineering, for example, has
gained significant success because of the relative simplicity
of the tissue architecture. Other tissues with extensive vasculature and heterogeneous cell arrangements, however,
present significant hurdles in creating functional tissues that
mimic their physiological counterparts.5,6 A fundamental
understanding of the mechanisms and regulatory factors
involved is required for creating functional tissue constructs
for regenerative medicine.
The chemical basis of biological pattern formation and
tissue morphogenesis is best understood in the reactiondiffusion model, which is also referred to as the
activator-inhibitor system or the Turing pattern.7 Moreover,
the theoretical model describes the roles of autocatalytic reactions and lateral inhibition in tissue morphogenesis and has
been applied to explain a wide spectrum of morphogenic processes.8–11 In addition to the reaction and diffusion of morphogens, physical factors are also important throughout the
developmental process. The regulatory role of cell mechanics
(e.g., cell contractility, intercellular tension, and cell-matrix
mechanical interactions) is increasingly recognized in tissue
morphogenesis.12 As an example, geometric confinement has
been demonstrated to regulate capillary network topology via
cell-matrix mechanical interactions.13 Tissue deformation has
been shown to modulate vascular endothelial growth factor
1
Department of Aerospace and Mechanical Engineering, The University
of Arizona, Tucson, AZ, USA
2
Biomedical Engineering and Bio5 Institute, The University of Arizona,
Tucson, AZ, USA
3
Department of Medicine, The University of Arizona, Tucson, AZ, USA
*These authors contributed equally.
Received Jun 7, 2014.
Corresponding Author:
Pak Kin Wong, Biomedical Engineering and Bio5 Institute, The University
of Arizona, 11130 Mountain Ave., Tucson, AZ 85721 USA.
Email: pak@email.arizona.edu
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Figure 1. Biochemical
approaches for
perturbing cell
mechanics.
gradients and endothelial cell proliferation in deformable tissue constructs, coupling biochemical and mechanical tissue
regulation.14 Cell traction force has been demonstrated to control capillary network formation in vitro and in vivo by applying a Rho inhibitor and modulating extracellular matrix
(ECM) elasticity.15 Alternation of the ECM composition also
revealed the importance of endothelial cell traction force in
network stabilization.16 These studies highlight the mechanical basis of tissue morphogenesis and encourage the development of novel mechanoregulation techniques.
Cells in tissues interact with their physical environment
and generate endogenous contractile forces via multiple
feedback mechanisms.17–19 The cell traction forces are coordinated by the cytoskeleton dynamics as well as cell-cell
and cell-ECM interactions. Advances in biochemistry,
material science, microfabrication, and nanotechnology
have created new opportunities in modulating these interactions and probing the mechanoregulation of tissue morphogenesis systematically.20–24 In this article, we summarize
advances in techniques and approaches for studying the
roles of mechanical factors in tissue morphogenesis. First,
pharmacological and biochemical approaches for perturbing the cytoskeletal structures and cell adhesion molecules
are discussed. Second, biomaterial and microfabrication
techniques for modulating the cell-matrix mechanical interactions are presented. External physical perturbations with
mechanical, optical, magnetic, and fluidic techniques are
then reviewed. Examples of the development of vascular
and other tissues are described to illustrate the applicability
of these techniques. Finally, the potential and limitations of
these techniques for probing the mechanoregulation of tissue morphogenesis are discussed.
Biochemical Approaches for
Mechanical Perturbation
Actin-Targeting Reagents
Cell traction forces and intercellular mechanical interactions can be modulated by reagents that target various
cellular components such as actin filaments, microtubules,
actomyosin, and cell junctions. The chemical inhibitors,
RNA interference, forced expression, as well as genetic
mutations of cellular components have been used to alter
the mechanical properties of cells (Fig. 1). It is well known
that the actin cytoskeleton plays a major role in mechanical
support, generation of cell traction force, and transmission
of exogenous and endogenous cellular stress; therefore,
actin-targeting drugs possess a significant capability to perturb cell mechanics. For example, cytochalasins, which are
F-actin (filamentous) depolymerization reagents, bind to
the barbed end of F-actin and prevent actin monomer
polymerization. With cytochalasin D treatment, the contractile force and dynamic stiffness of endothelial cells are
reduced significantly, whereas the intracellular structural
damping remains unaffected.25 In angiogenesis studies,
endothelial cell tube formation is also inhibited by cytochalasin D.26 Interestingly, at low concentrations of cytochalasin D, changes in the actin cytoskeleton are undetectable by
fluorescence microscopy, despite significant changes in the
mechanical properties of the cells.27 Cytochalasin E, similar
to cytochalasin D, inhibits endothelial proliferation and
angiogenesis without disrupting actin stress fibers.28
Latrunculin, a class of actin depolymerizing agents,29,30
is more potent than cytochalasins. Latrunculin B binds to
G-actin (globular) and inhibits nucleotide exchange on
actin, while reducing cell stiffness by approximately 50%.31
Low concentrations of Latrunculin B preferentially inhibit
F-actin polymerization in filopodia.32 Y-27632, a synthetic
pyridine derivative that inhibits the Rho kinases (ROCK I
and ROCK II), affects various downstream events including
F-actin depolymerization and inhibition of myosin activity.
As a contraction inhibitor, it relaxes contraction in smooth
muscle cells and trabecular meshworks.33 Treatment with
Y-27632 abrogated vascular endothelial growth factor
(VEGF)–induced softening of human umbilical vein endothelial cells in three-dimensional (3D) matrixes34 and inhibited angiogenesis in vivo without observable effects on
preexisting vessels.35 In addition to chemically regulating
the actin cytoskeleton, RNA interference of actin can also
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be applied to modulate mechanical forces. For example, the
interference of smooth muscle actin (α-SMA) attenuated
transforming growth factor β (TGF-β)–induced cell traction
force in fibroblasts, and a linear relationship was also
observed between the α-SMA protein expression level and
the magnitude of cell traction force.36
Microtubule-Targeting Molecules
Microtubule-targeting drugs represent another group of
potent agents for modulating cell mechanics; however, the
mechanical effects of microtubule-targeting drugs are often
ultimately related to the actin cytoskeleton.37 Nocodazole, a
synthetic benzimidazole that destabilizes microtubules, disrupts the cellular balance between actin and microtubule
networks and indirectly activates the actin cytoskeleton to
improve force generation.38 In addition, nocodazole exhibits anti-angiogenic effects in vitro along with potent inhibitory effects on endothelial growth.39 Taxol (paclitaxel), an
agent that stabilizes microtubules, facilitates microtubule
polymerization and increases cell stiffness and viscosity.40
Taxol has been actively investigated for its anti-angiogenic
effects both in vitro and in vivo.41 Interestingly, taxol, in
low concentrations, has been shown to inhibit angiogenesis
in vitro without affecting microtubule assembly.42
Drugs Targeting Actin-Myosin Interactions
The modulation of cell mechanics in physiological and pathological processes is also achieved by controlling actin-myosin
interactions. Myosins are a family of ATP-dependent motor
molecules responsible for cell contraction and motility in both
muscle and nonmuscle cells. For instance, 2,3-butanedione
monoxime, an inhibitor of the ATPase activity of myosin,
depressed the contractile characteristics of muscle cells43 and
decreased the stiffness of cardiomyocytes.44 ML-7 and ML-9,
agents that bind to myosin light chain kinase competitively
with ATP, can also be used to reduce cell stiffness.45,46
Blebbistatin, a pharmacological inhibitor that restricts actinmyosin interaction by lowering myosin’s affinity for actin,
reduces contractile force and cadherin adhesion.47
RNA interference can be applied to inhibit actomyosin
interactions and angiogenesis.48 However, myosin inhibition through RNA interference also influences the mechanical properties of the cells. Myosin IIA small interfering
RNA has been demonstrated to markedly reduce the contraction force of fibroblast cells within fibrins.38 The inhibitor of an unconventional myosin, Myo1G, was also shown
to decrease the elasticity of Jurkat cells significantly.49
Targeting Intermediate Filaments
Another major component of the cytoskeletal system is the
intermediate filament network. Intermediate filaments play
essential roles in providing mechanical and structural integrity for cells, including regulating cellular tension development.50 Multiple types of intermediate filament proteins
have been identified. Among them, vimentin is one of the
most widely distributed intermediate filament proteins.
Vimentin intermediate filaments support cellular membranes, fix the position of some organelles, and transmit
membrane receptor signals to the nucleus. Stimulation of
tracheal smooth muscle strips with acetylcholine induced
the increase in the ratio of soluble to insoluble vimentin in
association with force development.51 Treatment of muscle
tissues with vimentin RNA interference attenuated force
development in response to acetylcholine or KCl depolarization, as well as lower passive tension; the latter may be
associated with the impairment of desmosomes.52
Targeting Focal Adhesions and Cell Junctions
Cell mechanics may also be modulated by targeting cellmatrix and cell-cell interactions with genetic manipulation.
Vinculin, which couples integrins or cadherins to the actin
cytoskeleton at focal adhesions and adherens junctions, has
been frequently used for assessing cell mechanical measurements. Cell contractile force generation is reduced
when vinculin is absent or enhanced when vinculin is upregulated.53 Depleting the paxillin-vinculin interaction by
substituting endogenous paxillin with a mutant reduced the
total traction force of mouse embryonic fibroblasts.54
Overexpression of the vinculin binding domain of αEcatenin decreased E-cadherin–mediated adhesion strength.55
Down-regulation of talin I, an adaptor protein that links
integrins to actin at the adhesion complex, decreased cellular force generation.56 Depletion of α-actinin, which links
integrins with actin, enhanced initial force generation and
prevented adhesion mutation in subsequent steps.57
Overexpression of actin-binding protein caldesmon blocked
cell contractility and interfered with focal adhesion formation.58 Chemical inhibitions of focal adhesion kinase with
specific inhibitor or broad spectrum inhibitor have been
shown to increase traction forces.59 Activation of integrins
with integrin-activating antibody inhibited the contractile
force of muscle cells. Similar effects can be achieved by
treatment with a synthetic integrin-binding peptide, whereas
integrin function-blocking antibodies reversed the effect of
the peptide on contractile force.60
Targeting Signaling Pathways That Regulate Cell
Mechanics
The manipulation of several signaling pathways can directly or
indirectly lead to changes in cell mechanics. RNA interference
of transcription factor JunB led to the inhibition of cell contractility under both basal and TGFβ1-stimulated conditions.61
Treatment of DNA binding protein HMGB1 caused a
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Table 1. Common Gel-Based Materials for Studying Cell-Matrix Mechanical Interactions.
ECM Type
ECM Component
66,67
Ligand Density
Stiffness Range
Model and Parameters
Protein
Hydrogel
Fibrin
2.5–10 mg/mL
1.3–9.0 kPa
Half synthetic
Hydrogel
Fully synthetic
Hydrogel
Glycated collagen69
1.5 mg/mL
175–515 Pa
PA/collagen70
0.1 mg/mL
0.2–10 kPa
HUVEC coated beads
Total length and
number of sprouts
BAEC spheroids
Same as above
Capillary-like structures
Collagen/PA/glass
Sandwich71
Peptide68
1.6 mg/mL
0.3–14 kPa
Pseudopodial branching
1–3 mg/mL
46–735 Pa
Degradable
PEGDA72
10 mg/mL
3.17–0.62 kPa
Capillary-like network
Average structure size
Vascular sprout
Direction
Effects
Decreased
3-fold
Decreased
2- and 1.5-fold
Form on ECM
Stiffness <1 kPa
Inhibited
Fewer branches
Decreased
474–60 µm
Stiffness
Dependent
BAEC, bovine aortic endothelial cell; ECM, extracellular matrix; HUVEC, human umbilical vein endothelial cell; PA, polyacrylamide; PEGDA,
polyethylene glycol diacrylate hydrogel.
TLR4-dependent increase in traction force, accounting for the
downstream inhibition of enterocyte migration.62 Silencing of
the function-unknown gene VPS13A attenuated the F-actin
network and reduced the stiffness of endothelial cells.63
Silencing integrin-linked kinase enhanced vascular smooth
muscle cell contraction under a puling force.64 Inhibition of
soluble adenylyl cyclase results in significant endothelial cell
softening.65 However, for applying biochemical approaches in
studying cell mechanoregulation, achieving a pure mechanical
perturbation is still a challenge because of the complex signaling network. Analyzing the results due to the force feedback–
sensing, as well as force feedback–generating, mechanisms in
cells is also demanding.
Biomaterial Approaches for
Modulating the Microenvironment
ECM and Substrate Properties
Cell-matrix interactions can be modulated by adjusting the
matrix properties (e.g., stiffness) as well as by altering ECM
geometry and/or topography (Table 1). The stiffness of the
matrix can be controlled by the ECM density. Increasing the
density of fibrin from 2.5 to 10 mg/mL resulted in an
approximately sevenfold increase in matrix stiffness.66
Fibrin gel has been used to study 3D capillary morphogenesis, with stiff gels found to reduce the formation of capillary networks.16,67 Similarly, the extent of capillary-like
network formation could be tuned by adjusting the density
of a self-assembling peptide gel.68 Despite the simplicity of
this approach, adjusting the matrix stiffness via the gel density will also affect the ligand density, which could introduce ambiguity into the results.
Glycation of ECM proteins (e.g., type I collagen) has
been employed to increase ECM stiffness with constant
ligand density. More specifically, collagen gels were incubated in glucose-6-phopshate for 5 and 8 d to achieve different levels of glycation and ECM stiffness. Using collagen
glycation, sprouting angiogenesis was shown to be delayed
in stiff gels in a co-culture (endothelial cells and smooth
muscle cells) sprouting model.73 Alternatively, collagen
solutions could be mixed with ribose to form glycated collagen solutions with increased concentrations of ribose and
stiffness (~175 to ~730 Pa), leading to an increased number
and length of sprouts from endothelial cell spheroids.69
However, the time for complete glycation is long and the
range of comprehensive moduli is small in comparison to
the relevant physiological range (hundreds of kPa).
Biological inert synthetic polymers have also been developed for achieving tunable gel stiffness with constant ligand
density.74,75
Polyacrylamide (PA) hydrogels may achieve a large
range of ECM stiffness by simply altering the relative concentrations of monomer (acrylamide) and cross-linker (bisacrylamide).76 However, it is not compatible with cell
culture directly because of potential toxicities as well as the
lack of cell adhesion capability. Proper PA hydrogel modification is required for investigating the effects of mechanical
interactions at the cell-substrate interface. Endothelial cells
have been shown to self-organize into capillary-like structures on compliant PA gels derivatized with type I collagen
or functionalized with Arg-Gly-Asp (RGD) peptides.70,77 A
collagen/PA/glass sandwich gel that enabled 3D cell culture
was also shown to manipulate substrate stiffness, revealing
that high ECM stiffness inhibits endothelial pseudopodial
branch initiation.71
Micropost arrays have also been used for modulating
substrate rigidity, by taking advantage of microfabrication
technology. The micropost arrays have been primarily used
for studying cell mechanical interactions on 2D substrates
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and can be modified for studying 3D microtissues.78 The
deformability of the micropost can be modified by adjusting the post dimensions (e.g., height). Micropost arrays
allow independent control of surface properties, decoupling
substrate rigidity from cell adhesion.79,80 Furthermore, for
compliant microposts (e.g., polydimethylsiloxane [PDMS]),
cell traction forces can be determined by measuring the
micropost displacement. It has been reported that endothelial cells showed enhanced elongation and alignment on
PDMS microposts compared with stiff SiO2 microposts
with similar topographical features.81 However, the effects
of ECM roughness and topology cannot be ignored and may
limit micropost applications on stiffness-associated cell
research.
Dynamic tuning of local ECM stiffness was demonstrated with polyethylene glycol (PEG)–based photodegradable hydrogels, in which cells exhibited a rounded
morphology initially and started spreading during irradiationinduced matrix stiffening.82 In addition, light-mediated
sequential cross-linking was applied for dynamic matrix
stiffening (e.g., 3–30 kPa), which mimicked the dynamic
nature of tissue development, wound healing, and pathogenesis.83 Increased cell areas and traction forces were
observed in adhered human mesenchymal stem cells over a
time scale of hours as the substrates were stiffened.
PA hydrogels with a photo initiator have been used for
manipulating spatial control of the matrix stiffness. Stiffness
gradients were generated by progressively uncovering the
gel solution with an opaque mask from a noncollimated
ultraviolet lamp.84 Alternatively, a matrix metalloproteinase
(MMP)–sensitivity PEG diacrylate hydrogel (PEGDA) was
used to generate stiffness gradients via matrix degradation.72 The stiffness gradients were capable of directing 3D
vascular sprout formation using a co-culture angiogenesis
model. In addition, magnetic beads could be embedded in
collagen gels via bio-conjugation to alter the local stiffness
of the ECM at the presence of an external magnetic field.85
The magnetic force could increase the apparent stiffness of
the ECM, and magnetic force gradients could induce ECM
stiffness gradients, which can affect endothelial cell behaviors during angiogenesis.
Geometric Control
ECM geometries and physical confinements created by lithography and other microfabrication techniques can also modulate
tissue morphogenesis.86,87 Photolithographic techniques were
applied for micropatterned PEGDA with adhesive ligands to
regulate and guide endothelial morphogenesis.88 Endothelial
cells did not assemble into cordlike structures on the stripe that
was larger than 50 µm in width, highlighting the importance of
geometric control of endothelial morphogenesis. Geometric
control of endothelial cord formation was also demonstrated
by culturing cells in microchannels that were filled with
collagen gels.89 The microchannel confinement is also used for
directing the development of branches in tube formation,
which may enable the production of complex capillary architectures. More recently, endothelial cells cultured on narrow
(10 and 50 µm) micropatterned angiogenic Ser−Val−Val−
Tyr−Gly−Leu−Arg (SVVYGLR) peptides demonstrated
restricted spreading, with orientation and migration directionally guided and regulated.90 Microwells with arbitrary shapes
(e.g., star, square, and triangle) were shown to module capillary topology via cell-matrix mechanical interactions.13
Endothelial cells can form denser networks on acute angle corners than those on reflex angle corners in a star-shaped ECM
structure. The geometric control of tissue morphogenesis has
significant implications in microfabrication-based tissue models and scaffold design for tissue engineering.
ECM topography and fiber alignment can also affect cell
behaviors, such as the orientation of actin filaments and
focal adhesions, proliferation, and migration.91–93 In particular, endothelial progenitor cells on nanotopographic substrates (600-nm-wide ridge and groove) exhibited enhanced
alignment, organization, and capillary tube formation.94
With the improvement and development of micro- and
nanofabrication technologies in the future, mimicking in
vivo ECM with specific topology for investigating tissue
morphogenesis will be possible.
Biophysical Approaches for Probing
Tissue Morphogenesis
Mechanical Perturbation
Mechanical perturbation influences various cell functions,
including proliferation, migration, differentiation, and ECM
remodeling (Table 2). Tissue deformation that occurs naturally
in muscular and cardiovascular systems regulates various tissue morphogenic processes.95,96 Mechanical stretching of rat
microvascular endothelial cells enhanced the production of
MMP-2, MT1-MMP, HIF-1α, HIF-2α, and VEGF at both the
transcriptional and translational levels.97 Stretching also
induced an increase in intracellular Ca2+ concentration and
traction forces of NIH-3T3 fibroblasts via stretch-activated ion
channels.98 In addition, physical stimulation with a mechanical
probe may also be applied to create local mechanical stress and
injury to individual cells in the tissue.99–101 A mechanical probe
with comb-drive (capacitive) force measurement, for instance,
has been used to study mechanical stimulation–induced intercellular calcium communication in microengineered endothelial networks.102,103 Endothelial cells collectively regulated
their calcium levels against a large range of probing forces and
repeated stimulations. Moreover, endothelial cells on elastic
gels were shown to have active calcium responses when stimulated with a vibrating probe.104 The mechanical stimulation
caused deformations over a distance from the probe, and the
cells sensed the local stimulation.
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Table 2. Biomechanical Effects on Tissue Morphogenesis.
Mechanical Perturbations
Mechanical stretch
Tissue or Cell Type
Approach
Rat microvascular endothelial
cells, Sprague-Dawley rats
Fibroblast
Overload rat extensor
digitorum longus muscles
Stretch elastic gel
Optical technique
HUVECs
Manipulate fibronectin-coated
beads
HMVECs
ECs, Drosophila embryos
Apply tensile and compressive
force
Laser ablation
Magnetic technique
HUVECs
Adjust magnetic fields
Smooth muscle cells
Drosophila embryos
Uniform-gradient magnetic
manipulator
Manipulate magnetic particles
Microposts
Magnetic microposts
Shear stress
Rabbit ear chamber, rat
microvascular endothelial
cells, Sprague-Dawley rats
BAEC, HUVECs, bovine
pulmonary microvascular
endothelial cells
Parallel plate flow chamber
Parallel-plate flow chamber
Tissue or Cell Response
Increased capillary number105,106
Increased leading edge
proliferation, proliferation
rate100,107
Rapid distal Src activation and
slower directional wave
propagation of Src activation108
Increased capillary growth109
Increase cell migration, neural
regeneration, morphgenic
movement110–112
Accelerate cell migration,
regulate migration direction113
3D cellular deformation114
Compression of stomodeal
cells115
Increase in local focal adhesion
size116
Increased capillary density,
capillary number, and capillary
growth rate105,106,117
Enhanced sprouting,118
directional assembly119 directs
cell sprouting120
3D, three-dimensional; BAEC, bovine aortic endothelial cell; EC, endothelial cell; HMVEC, human microvascular endothelial cell; HUVEC, human
umbilical vein endothelial cell.
Optical Techniques
Optical techniques can generate mechanical perturbations
with subcellular resolution. As an example, Src activities in
single endothelial cells could be activated mechanically by
laser tweezers with fibronectin-coated beads.121 Optical
tweezers in a microfluidic system generated tensile and
compressive forces for studying focal adhesion recruitment
in endothelial cells.122 In particular, mechanical force was
applied to the ECM-integrin-cytoskeleton linkage by trapping and manipulating functionalized beads. In addition to
laser tweezers, photoablation of cells and subcellular structures with lasers represented another powerful approach in
modulating cell mechanics with subcellular resolution.108
Cell ablation may be applied to disrupt the mechanical
interactions between cells and perturb specialized cells in
the morphogenic process. For disrupting the cytoskeleton,
fluorescent proteins, such as actin-GFP, were used to visualize the cytoskeletal structure, and lasers were applied to
ablate or disrupt the cytoskeleton. Femtosecond lasers with
multiphoton absorption enabled fine resolution (<300 nm)
and minimized damage to surrounding structures. The photoablation technique has been used for investigating various
tissue morphogenic processes, such as collective cell migration,109 neural regeneration,108 and morphogenic movements in Drosophila embryos.110
Magnetic Technique
Magnetic tweezers are employed to apply mechanical forces to
living cells and tissues via external magnetic field gradients.111
Intracellular forces on endothelial cells are generated under
different magnetic fields after cellular uptake of superparamagnetic iron oxide nanoparticles. The intracellular magnetic
force accelerates cell migration and regulates cell migratory
direction via adjusting magnetic fields and increasing the cellfree culture space.112 Moreover, 3D cellular deformation was
studied by applying uniform forces to a large cell population
via a uniform-gradient magnetic manipulator.113 Magnetic particles were injected into Drosophila embryos for in vivo studies of tissue deformation. The mechanical compression of
stomodeal cells in embryo mimicked the physiological deformation experienced by stomodeal cells due to germ band
extension at the onset of gastrulation, which could up-regulate
Twist expression in the stomodeal primordium.114 Magnetic
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microposts were also employed to exert a step force that led to
an increase in local focal adhesion size at the site of application
but not at nearby nonmagnetic posts.115
Shear Stress
Endothelial cells are subjected to shear stress generated by
blood flow. Shear stress can modulate cell migration, proliferation, gene expression, and morphology of endothelial
cells.116,123 Thus, wall shear stress can be generated to investigate the effects of dynamic shear on the development and
remodeling of the vasculature. A parallel plate flow chamber may be used to study the effects of shear stress on capillary-like tube formation. Endothelial cells on matrigel
experienced high shear stress, resulting in aligned tubular
structures along with the direction of fluid flow. Shear
stress–dependent regulation was, at least partially, induced
by VEGF expression.105,117,120,124 Fluid forces generated by
shear stress may also attenuate endothelial cell sprouting in
a nitric oxide–dependent manner. Interstitial flow directs
cell sprouting and cell morphology and therefore regulates
capillary tube formation. Moreover, fluidic shear stress (3
dyn/cm2) enhanced the sprouting of bovine pulmonary
microvascular endothelial cells in a 3D culture model.120 In
vivo, the effects of fluid shear stress in capillary vessel
growth were first investigated in rat skeletal muscle and
rabbit ear chamber.106,118 Externally applied wall shear
stress was shown to stimulate angiogenesis via induction of
VEGF expression by HIF-1α via the PI3K-dependent Akt
phosphorylation pathway.
Microfluidic technology provides powerful tools for
investigating the effects of fluidic shear stress on tissue
morphogenesis in 2D and 3D models.125 Fluid shear can be
applied to perturb diffusible gradients in the microenvironment, thereby isolating biomechanical mechanisms from
other biochemical mechanisms (e.g., reaction-diffusion).
For instance, microfluidics was used to identify autocatalytic alignment feedback in the long-range organization of
myotube development.126 While diffusible factors are
essential in the alignment process, recirculation of the
media results in normal cell fusion and long-range alignment, eliminating diffusible factors from the cell alignment
mechanism. Furthermore, a microfluidic wound-healing
assay was developed to wound the monolayer and test the
effects of shear stress on cell migration.127
Discussion
This review summarizes biochemical, biomaterial, and biophysical approaches for probing the regulatory roles of
mechanical force in tissue morphogenesis. Several global
perturbation techniques, such as modulation of ECM properties, fluid shear, and mechanical stretching, have been
used extensively because of their physiological and
translational relevance. These techniques are particularly
useful in identifying the involvement of mechanical forces
in morphogenic processes. The overall tissue architectures
(e.g., orientation and network density) can be perturbed to
shed light on the morphogenic process. Advances in technology have also enabled local perturbation techniques,
such as single-cell probes, laser tweezers, microfluidics,
and photoablation, for investigating cell-cell communication mechanisms. These techniques open new possibilities
in elucidating morphogenic mechanisms by perturbing the
local mechanical force distribution and observing its effects
on the tissue architecture. In particular, these local perturbation approaches can be used for exploring unknown morphogenic processes, testing new hypotheses, and studying
cell-cell organization. Furthermore, biochemical reagents
provide invaluable insights into the molecular mechanisms
involved in the mechanoregulation processes. However,
biochemical reagents may disturb multiple signaling pathways and trigger nonspecific effects, which complicates the
interpretation of results. Multiple reagents and approaches
should be incorporated to confirm that the observation is
indeed mechanically induced.
Cells and tissues represent complex networks of mechanically responsive systems with multiple hierarchical levels.
Cells in tissues collectively sense and adapt to the physical
microenvironment during development and regeneration.
Intra- and intercellular forces are dynamically regulated
through multiple cellular feedback mechanisms. While it is
important to understand the molecular mechanosensing
mechanism, it is challenging, if not impossible, to isolate
individual factors one from the other. This represents a hurdle in understanding and interpreting study results and in
elucidating mechanical regulatory mechanisms. Another
challenge in understanding tissues morphogenesis is to
identify the emerging behaviors that drive complex tissue
architectures. Inhibiting a morphogenic process does not
necessarily provide useful information in the conceptual
understanding of the emerging behaviors. Careful experimental designs with multiple techniques are often required
to study collective cell behavior. In addition to mechanical
perturbation techniques, novel biochemical and mechanical
techniques (e.g., intracellular probes100,128 and traction force
microscopy129,130) should be developed and used to study
morphogenic processes. Another key challenge in studying
the emergent complexity of biological tissues is the difficulty in understanding the high-order structures and functions resulting from local interactions of individual cells.
These emergent properties can often be counterintuitive and
cannot be understood by simply extrapolating interaction
between a few cells. The complex interplays between global
microenvironmental cues and local cell-cell interactions
further complicate understanding emergent cell behavior. A
complex systems framework that incorporates biomanufacturing, microfluidics, advanced materials, biosensors, and
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computational modeling is ultimately required to fully
understand the mechanoregulation of tissue morphogenesis.
As we move forward in this field to understand this complexity, not only will basic cellular and tissue processes
become clear, this insight may additionally be used in a biomanipulative way to develop novel therapeutic strategies to
tackle complex disease processes that involve mechanical
regulatory mechanisms.
Acknowledgments
The authors would like to thank Zachary Dean for constructive
suggestions and English editing.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect
to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: This
work was supported by National Institutes of Health Director’s
New Innovator Award (DP2OD007161).
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